Split nanofocusing spots beyond diffraction limit via a new near-field plasmonic structure

The focusing spot beyond diffraction limit is critical to plasmonic direct-writing lithography. To improve the speed and precision of plasmonic direct-writing lithography, we design a new periodically repeated circular hole/elliptical ring plasmonic structure named as split-focusing structure used for producing two focusing spots under the incidence of linearly polarized plane wave at 633nm wavelength. It consists of SiO₂ infstrate and coated silver film with holes and slits of different shapes. By designing appropriate structure parameters to excite localized surface plasmon resonance, two split infwavelength spots are produced on the focal plane. Finite-difference time-domain (FDTD) method is used for numerical simulation. The simulation result indicates that the focal length of structure is 36nm and the full width at half maximum (FWHM) of single spot is 50nm. Both split spots can be used for direct writing so the speed of photoetching will be raised. The dual spots are both in circular shape, which is beneficial to improve the pattern precision. The influence of structure parameters on focusing performance is also analyzed to guide the practical fabrication of structure. The split-focusing structure designed in this paper also owns application values in data storage and non-contact sensing.